I asked around a little about the R22 rotor design after reading Frank's post and the numerous others on the subject and I heard pretty much the same thing that Frank said. The 18 degrees was designed into the head to try to deal with some complex characteristics involved with pitch coupling. The R22, if designed with a 90 degree offset, would force the pilot to compensate in the low speed regime. The design as it stands, necessitates some compensation at higher speeds. From what I understand, without complex flight control servos and such, the design chosen by the manufacturer will always be a compromise. I'd love to hear more on the subject myself, especially from someone who has a knack for teaching.